[Haskell-cafe] Fail-back monad

[Alberto G. Corona]

But, on my side,
 BackT [BackPoint 1]
 and
 BackT [NoBack 1]

can be made indistinguishable outside the code where the monad is
defined.  Probably the effect required is not possible without
breaking the law

2012/3/31 Ross Paterson <ross at soi.city.ac.uk>:
> On Wed, Mar 28, 2012 at 01:34:29AM +0100, Alberto G. Corona  wrote:
>> In my package MFlow [1] I program an entire web  navigation in a
>> single procedure. That happened  in the good-old WASH web application
>> framework.
>> The problem is the back button in the Browser.
>> To go back in the code to the previous interactions when the data
>> input does not match the expected because the user pressed the back
>> button one or more times, i came across this Monad specimen,: that
>> solves the problem.
>
> This definition does not satisfy the right identity law (m >>= return = m)
> included in the monad definition:
>
> *FailBack> BackT [BackPoint 1] >>= return
> BackT [NoBack 1]
>
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